1. Field of the Invention
This invention relates to an air-fuel ratio control method for internal combustion engines, and more particularly to a method of controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine when the engine is in a high load operating condition.
2. Prior Art
It is conventionally known to control the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (hereinafter referred to as "the supply air-fuel ratio") to a stoichiometric air-fuel ratio or its vicinity when load on the engine is relatively low, and enrich the supply air-fuel ratio to prevent the temperature of the engine from rising to an excessive degree by utilizing the effects of cooling by fuel in the air-fuel mixture supplied to the engine, when the load on the engine is high. To carry out this air-fuel ratio control method, the following techniques have conventionally been proposed:
(1) A desired exhaust gas temperature is set based on the amount of intake air, engine rotational speed, and engine coolant temperature, and the supply air-fuel ratio is controlled such that the actual exhaust gas temperature becomes equal to the set desired exhaust gas temperature (Japanese Provisional Patent Publication (Kokai) No. 60-90940).
(2) An exhaust gas temperature is estimated based on the amount of intake air or engine rotational speed and the supply air-fuel ratio is enriched to a greater extent as the estimated exhaust gas temperature is higher (Japanese Patent Publication (Kokoku) No. 62-54977).
(3) The temperature of a catalytic converter provided in an internal combustion engine is estimated based on the amount of intake air and the supply air-fuel ratio, whereby the supply air-fuel ratio is controlled so as to prevent an excessive rise in the temperature of the catalytic converter (Japanese Provisional Patent Publication (Kokai) No. 62-203965).
(4) An engine temperature is estimated based on engine rotational speed, load on the engine, and the supply air-fuel ratio, and enriching of the supply air-fuel ratio is controlled depending on the estimated engine temperature (Japanese Provisional Patent Publication (Kokai) No. 3-18643).
Further, to determine whether an oxygen concentration sensor arranged in an exhaust passage of the engine is activated or not, the following technique has also been proposed:
(5) The temperature of the oxygen concentration sensor is estimated based on the amount of intake air and outside air temperature (Japanese Provisional Patent Publication (Kokai) No. 1-219340).
According to the above techniques (2) to (5). the exhaust gas temperature or the temperature of an engine component part is estimated based on engine operating parameters, such as the amount of intake air and engine rotational speed, but the actual exhaust gas temperature is not detected. Therefore, there is a possibility of an estimated value of the exhaust gas temperature becoming largely different from an actual value of same. To overcome this disadvantage, it is required to make wider the engine operating region in which the supply air-fuel ratio should be enriched (hereinafter referred to as "the high load enriching region"), i.e. set a reference temperature for determining whether the supply air-fuel ratio should be enriched to a lower value. As a result, there can be cases where the air-fuel ratio is unnecessarily enriched, which results in degradation of fuel consumption and exhaust emission characteristics.
Further, according to the above technique (1), the temperature of the exhaust system is determined only by detecting the exhaust gas temperature, and the desired exhaust gas temperature is set based on the amount of intake air, engine rotational speed, intake air temperature, and engine coolant temperature to control the supply air-fuel ratio such that the detected exhaust gas temperature becomes equal to the desired exhaust gas temperature. However, the temperature of engine component parts, which may rise to an excessive degree, varies not only by the exhaust gas temperature but also by the volume of hot exhaust gases. More specifically, even if the exhaust gas temperature remains unchanged, the rate of rise in the temperature of engine component parts tends to be lower when the volume of exhaust gases (which may be determined by the engine rotational speed and engine load) is smaller than when the volume of exhaust gases is larger. In the case of a three-way catalyst, for example, it has been found that even if the exhaust gas temperature remains unchanged, when the engine is in a high load and high engine rotational speed condition, in which the volume of exhaust gases is larger, the rate of thermal conduction to the three-way catalyst tends to increase due to the flow of an increased volume of hot exhaust gases to cause the temperature of the three-way catalyst to rise at a higher rate. On the other hand, when the volume of exhaust gases is smaller, the temperature of the three-way catalyst rises at a lower rate in spite of the presence of the flow of hot exhaust gases. Further, it has also been found that due to difference in thermal capacity between engine component parts, the amount of thermal conduction therethrough varies with the engine component parts, which causes the temperatures of the engine component parts to rise at different rates as time elapses. Therefore, there is room for improvement of this technique (1) concerning fuel consumption and exhaust emission characteristics.